I don't know what you have.
I don't know what you do.
I don't know what you see.
It is really, Really, REALLY difficult to fix a problem that can not be seen.
use COPY & PASTE so we can see what you do & how Oracle responds.

I do a full export on the 10g database (RAC): expdp with no other special parameters except for the necessary files (output, log).
After that I do a normal import in a newly created 11g database (RAC): impdp with again no special parameters except for the necessary files (input, log).

The log file of the export contains no errors except for compilation errors on the packages because of missing grants to sys objects. But no errors reported about constraints, tables or other objects.

When I check the imported objects (through the view all_objects) I count 4000 constraints less then on the source db, all 4000 are nameless check constraints.

Please understand the following statement is true.
Different Oracle versions contain different objects.

You have a few alternatives available to you.
1) Ignore this observation & concentrate on actual problems & errors.
2) File a bug report with MOS
3) run in circles & scream & shout that the Sky is Falling.